mail/fetchmail: update to 6.4.24 and block LibreSSL.

fetchmail cannot legally be linked with LibreSSL,
because there is no GPLv2 clause 2b exemption for
LibreSSL, only for OpenSSL.

Correct LICENSE and remove LICENSE_COMB.
Remove LibreSSL patch.

Add FSF comment suggested by Corey Halpin in PR.

Related to:
PR: 259214

Update:
PR: 259945
MFH: 2021Q4

Approved by: chalpin@cs.wisc.edu (maintainer)